QUESTION: My house was built in 2008. I have an HRV system and a high-efficiency furnace. My question is do I need to set the furnace blower/fan into an auto or on mode during the heating/cooling season? My HVAC technician says to run it continuously. However, as the HRV system runs continuously, why do I need to run the furnace blower continuously? What would be the potential problem if I put my furnace fan in an auto mode? I will appreciate your response. Prabal Ghosh
ANSWER: There are two elements that should be addressed -- the thermostat and fan-control setting for the furnace and the HRV. While these may seem like separate issues, they are often quite intertwined.
The answer to your direct question will depend on several factors, particularly the current settings of your Heat Recovery Ventilator (HRV). You're certainly not alone in your confusion about the proper way to use the controls for this ventilation device. To further complicate the issue, I've seen contradictory instructions in various homeowner manuals. One manufacturer may suggest intermittent cycling of the HRV, while another one may recommend continuous operation year-round. I'll try to explain the various options.
Since your home is relatively new and is likely well insulated and air/sealed, adequate ventilation is critical to maintaining healthy indoor air quality. Because of this, several manufacturers recommend running the HRV on low speed continuously. This may also be required to meet the ventilation standards of local building officials.
During the heating season, the HRV works to bring in clean air as well as dehumidifying stale, moist indoor air by replacing it with dry cool stuff from outside. This prevents condensation on cool windows and other surfaces, which can lead to mould growth. In my opinion, winter moisture control is the primary reason for installation of an HRV in a modern home.
What I don't understand is using a mechanical device to bring in exterior air that has a significantly higher moisture content than indoors during other seasons. In the hot summer months, this is likely the situation. We may spend a considerable amount of money on air-conditioning the indoor air, which dries it out considerably, only to pull in more damp air with the HRV in the name of adequate ventilation. In the summer, we are much more likely to enter and exit the home several times through the doors, which may provide adequate amounts of make-up air through natural ventilation.
In the shoulder seasons, when the outside temperature is moderate, leaving several windows open will easily provide enough ventilation for a normal home. For this reason, I don't think the continuous setting should be used, except in the winter.
A more frequent problem I often see with many HRV installations is the reliance on manual switches that operate only on a continuous on/off setting. This is the proper operation for switches in bathrooms that to remove excess moisture, but not at the main control. The main control should have an option for continuous use, but there should also be a functional humidistat which will operate when the continuous switch is off or on a low setting. This humidistat should work by turning on the HRV to a high setting when the Relative Humidity (RH) in the indoor air exceeds the setting.
In this way, the HRV will operate only when needed to dehumidify the indoor air while also removing indoor air pollutants. When the RH falls below this adjustable setting, which can be adjusted according to the outside temperature, the machine will shut off.
It's a similar situation with running your furnace fan continuously. It may not make sense to have your furnace circulating air continuously in your home on a temperate day when the windows are wide open. Even a slight wind should be enough to provide reasonable air-pressure differences for good ventilation on those occasions.
When the windows and doors are shut tight, and either the furnace or air conditioner is in use, continuous low-speed use of the furnace fan is prudent. This will improve indoor air circulation and prevent condensation from stagnant air when the thermostat is not calling for the furnace or A/C to operate.
Your HVAC technician is giving you good advice to run your furnace fan continuously when the HRV is operating. The fan and motor inside the HRV is much smaller and less-powerful the ones in the furnace. Because both of these appliances are connected to the return air ducting, running both fans at the same time will dramatically improve the airflow through the home when the HRV is operating. This should reduce the running time for the HRV on high speed, conserving energy and extending the life of the HRV motor.
What should also be included is a cross-link between the furnace fan and the HRV controls, so the furnace fan comes on at high speed when the humidistat exceeds its setting. This is when the maximum fan speeds should be used, to push the moist air through the home and ducts and replace it with cool, dry, exterior air.
Running the furnace fan on continuous low speed is a good idea to improve air movement within your home, but it only makes sense in the heating and cooling seasons. Unless you have areas in your home that are problematic for condensation and poor air circulation, seasonal use of this and the HRV should be sufficient.
Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home & Property Inspectors - Manitoba (www.cahpi.mb.ca). Questions can be e-mailed to the address below. Ari can be reached at (204) 291-5358 or check out his website at www.trainedeye.ca.